This application is based on Application No. 394602 filed in Japan on Dec. 26, 2001, the content of which is incorporated hereinto by reference.
This invention relates to a method of accurately computing remaining battery capacity and to a battery pack containing a circuit for that computation.
Electrical equipment which displays computed remaining battery capacity is convenient for the user. This is because the user can estimate remaining usable time while operating the electrical equipment. It is particularly important that this type of electrical equipment display remaining battery capacity as accurately as possible. If remaining capacity is displayed in error, inconvenient situations result. For example, electrical equipment may become unusable even though remaining capacity is displayed, or electrical equipment may be usable with no remaining capacity displayed.
Remaining capacity of a battery can be displayed in units of Ah (Ampere hour) by an integrated current method, or in units of Wh (Watt hour) by an integrated power or energy method. When battery voltage decreases in the Ah remaining capacity display method, it is necessary to compensate for that decrease when converting to consumed power. For example, if remaining capacity drops to 50% of full-charge capacity, remaining capacity displayed in Wh will not be 50% if battery voltage decreases. As a result, even if remaining capacity of a battery attached to electrical equipment consuming a constant amount of power is displayed as 50% in Ah, one half of the total usable time does not remain for use. This is because the amount of current to supply a constant amount of power must increase when battery voltage decreases. On the other hand, when remaining capacity is displayed as 50% of full-charge capacity by the Wh method of display, one half of the usable time remains for battery attachment to electrical equipment consuming a constant amount of power. Namely, the method of displaying remaining capacity as Wh of integrated power or energy has the characteristic that remaining usable time can be accurately inferred even if battery voltage decreases.
To compute remaining battery capacity based on integrated power or energy, discharge capacity calculated from integrated power is subtracted from the full-charge state, or integrated power discharge capacity is subtracted from charge capacity. Discharge capacity, which is integrated power or energy, is calculated by the time integral of the product of discharge current, voltage, and discharge efficiency.
Discharge capacity calculated by integrating the product of discharge current, voltage, and discharge efficiency is different from actual battery discharge capacity, and error results. Error between calculated and actual discharge capacity develops particularly easily when discharge current varies. This is because discharge efficiency varies widely with discharge current.
The present invention was developed to solve these drawbacks. Thus, it is a primary object of the present invention to provide a method of computing remaining battery capacity and a battery pack that can calculate discharge capacity more accurately and compute remaining capacity with a high degree of accuracy.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
The method of computing remaining battery capacity of the present invention calculates battery discharge capacity from the integral of the product of battery discharge current and voltage, and computes remaining capacity as integrated power or energy. The method of computing remaining capacity measures internal battery resistance, calculates battery discharge capacity by adding the value of integrated power consumed by internal battery resistance to the value of discharge capacity calculated from the integral of the product of battery discharge current and voltage, and computes remaining capacity from this discharge capacity.
The battery pack of the present invention is provided with a computation circuit 2 to calculate battery discharge capacity from the integral of the product of battery discharge current and voltage, and compute remaining capacity as integrated power or energy. The computation circuit 2 measures internal battery resistance, calculates battery discharge capacity by adding the value of integrated power consumed by internal battery resistance to the value of discharge capacity calculated from the integral of the product of battery discharge current and voltage, and computes remaining capacity from this discharge capacity.
The method of computing remaining battery capacity and the battery pack described above have the characteristic that discharge capacity can be more accurately calculated and remaining capacity can be computed with a high degree of accuracy. This is because the value of integrated power consumed by internal battery resistance is added to the discharge capacity value calculated from the integral of the product of battery discharge current and voltage to calculate battery discharge capacity. Since integrated power consumed by internal battery resistance is considered in the discharge capacity calculation, the error between actual discharge capacity and calculated discharge capacity can be drastically reduced, a more accurate discharge capacity can be calculated, and remaining battery capacity can be computed with a high degree of accuracy.
The method of computing remaining battery capacity and the battery pack of the present invention can compute remaining capacity by subtracting the value of integrated power consumed by internal battery resistance from the value of charge capacity calculated from the integral of the product of battery charging current and voltage. Since this method of computing remaining battery capacity and battery pack considers integrated power consumed by internal battery resistance for charge capacity computation as well as discharge capacity computation, it can more accurately compute remaining battery capacity.
This method of computing remaining battery capacity and the battery pack have the characteristic that not only discharge capacity, but also charge capacity can be more accurately calculated to allow remaining battery capacity to be computed with a high degree of accuracy.
Internal battery resistance can be calculated by measuring open circuit battery voltage, and charging voltage and current during battery charging. Further, internal battery resistance can also be determined by measuring discharge voltage and current during battery discharge.
The calculated value of discharge capacity can also be found from the integrated value of the product of battery discharge voltage, current, and discharge efficiency. Similarly, the calculated value of charge capacity can also be found from the integrated value of the product of battery charging voltage, current, and charging efficiency.